This invention relates generally to television receivers and particularly to the horizontal scansion systems thereof.
In the typical television receiver, a transmitted signal bearing information components of picture, sound and deflection synchronization information is received by an antenna and processed by a tunner and intermediate frequency amplifier to a level sufficient to permit recovery of the modulated components. The latter take the form of a combined train of pulses at vertical and horizontal scansion frequencies. A cathode ray tube (CRT) display device is caused to be simultaneously scanned in the vertical and horizontal directions by individual vertical and horizontal scansion systems within the receiver.
In both the horizontal and vertical systems a local oscillator generates a scansion signal which is increased by appropriate power amplifying circuitry to a level sufficient to drive electromagnetic yoke windings situated on the CRT. Proper display of the picture components within the signal requires that both the vertical and horizontal scansions be appropriately synchronized in frequency and phase to the incoming signal information. In the vertical deflection or scansion system (operative at approximately 60 hertz), the vertical scansion synchronization pulses are usually applied directly to the vertical oscillator triggering it and causing it to operate at the desired frequency and phase.
While this method of synchronization has proven satisfactory for the vertical scan system, the higher frequency (approximately 15 kHz), horizontal scansion system is generally synchronized in a different manner. In most horizontal scan systems a local automatic phase control loop is operative upon the oscillator. Such systems generally include a phase detector or multiplier which responds to the reference synchronization pulses and a feedback sample of horizontal output signal. The most common practice is to actually sample the horizontal retrace signal rather than the oscillator signal directly. The retrace signal is derived by the horizontal amplifier in cooperation with the horizontal output transformer and is a delayed-in-time signal at the same frequency as the oscillator output. The phase detector compares the reference and output signals and generate an error signal representing the deviation of the oscillator frequency and phase. The error signal is coupled to a low pass filter which minimizes the effects of signal noise and, in part, determines the response speed of the APC loop. The filtered error signal is coupled back to a voltage control point within the horizontal oscillator completing the loop and affecting frequency and phase control.
Automatic phase control systems operative upon the horizontal oscillator, are well known in the industry. However, while there use has proven generally satisfactory under mose operating conditions several often serious shortcomings do arise. For example, when a weak signal is received, the recovered synchronizing components include a great deal of noise. This results from the disproportionate amplification of thermal noise within the receiver's tuner and intermediate frequency amplifier when compensatory increases in system gain are made in response to lower signal levels. Under such conditions, APC system bandwidth results in jitter which appear as erratic back and forth shifting of portions of the displayed picture. It is desirable, therefore, that APC system bandwidth be narrow enough to eliminate or at least minimize such effects of noise components on the oscillator. However, narrowing the filter bandwidth increases the response time of the automatic phase control loop making it prohibitively slow. As a result the APC system often cannot respond quickly enough to correct rapid phase displacement which often occurs during the reception of video player signals.
In contrast, during strong signal reception the tuner and IF amplifier gain is reduced and the recovered signal components have lower noise content. With reduced noise in the signal, a wide APC filter bandwidth and a fast system response time are desired enabling it to compensate for problems of jitter and rapid phase displacements. The demands upon the APC system under differing signal conditions are conflicting and experience has shown they cannot be reconciled by selection of the filter time constant. As a result, most receivers are designed to compromise these conflicting system requirements.
The recent introduction of video players as a consumer item has further complicated the situation. It is not uncommon for video players, in particular the lower cost types most available to the consumer, to produce problems of jitter and abrupt phase displacements in the recovered horizontal synchronization information. This is generally due to the production of an abrupt phase displacement occurring during the vertical retrace interval, that is, between successive vertical scans. After a complete top-to-bottom scansion the CRT is retraced or indexed back to the top of the viewing screen. The difficulty generally arises due to the inability of the video player to maintain the phase of early horizontal synchronization pulses to those which last existed at the end of the previous vertical scan. As a result the horizontal scansion displayed at the top of the CRT is displaced or phase shifted.
Since the APC systems of most receivers represent a compromised speed of response to achieve adequate noise immunity they are unable to quickly compensate for such errors and require several horizontal scan lines to correct oscillator phase. The resulting condition manifests itself as a "hooking" or curving of vertical line picture elements in the upper portion of the displayed image.
The difficulty of resolving the conflicting performance requirements between broadcast and video player reception have prompted some television manufacturers to provide a switch which alternatively configures the receiver for either video player or broadcast signal reception by altering the bandwidth of the APC filter.
While such configurable APC filters provide some improvement in problems peculiar to video player operation, they do not solve the basic problem of conflicting APC system requirements, that is, the desire to have broad filter bandwidth high speed loop operation under some signal conditions and narrow bandwidth slow speed operation under other signal conditions.